| Product(s): | SewerGEMS, SewerCAD, CivilStorm, StormCAD |
| Version(s): | CONNECT Edition, V8i |
| Area: | Calculations |
Problem
How is the bend angle calculated in StormCAD / SewerCAD / SewerGEMS / CivilStorm?
How does the bend angle configured in calculating structure losses?
Background
The bend angle is the angle at which a pipe/s enter/s or exit/s a node. The angle is measured based on the relative orientation of the upstream and downstream pipe. The following sketch provides understanding on how the angle is measured.
The angle “X” can be measured with respect to the orientation of the downstream pipe (by extending the alignment of the downstream pipe and measuring the angle “X”).
Note: The bend angle should not be confused with modeling “bends”. Modeling “bends” for a pipe reduces the number of nodes by introducing bends in the pipe. This is just a layout functionality and these “bends” and not hydraulically considered to compute losses. If the user wishes to consider these the same can be defined as minor losses where different connections are available for selection.
The bend angle can be found in the pipe/conduit properties as “Bend Angle (Calculated)”. The user has the option of specifying the bend angle (if known) by setting the “Has User Defined Bend Angle?” to “True” and entering the value in the “Bend Angle (Calculated)” field. If kept “False” the software will automatically calculate the bend angle based on the geometry of the pipes in the layout after computing the model.
Although the bend angle is a property of the pipe the headloss introduced due to this is considered as structure losses at the node.
Note: The bend angle property of the pipe is available for the GVF-Convex, GVF-Rational and the Explicit SWMM Solvers only. The Implicit Solver available in SewerGEMS does not incorporate bend angle in the pipe properties.
The bend angle is important to calculate headloss through a node/manhole. There are several methods available to consider headloss through node. However, not all of them incorporate the bend angle in their calculation of headloss. Out of all the headloss methods available only the Standard, HEC-22 (2nd & 3rd Edition), AASHTO and the HEC-22 Minor Loss methods incorporate the bend angle into the calculation of headloss.
Solution
The angle the pipe makes with the node/manhole is sometimes accounted for in determining the headloss for that node. There are different methods to calculate headloss through the node (structure).
Standard Loss Method
A user-defined loss coefficient is used to calculate the head loss based on the velocity head of the exit conduit. The standard method calculates structure headloss based on the exit pipe's velocity. The exit velocity head is multiplied by a user-entered coefficient to determine the loss. This method incorporates the bend angle to determine the headloss coefficient to be adopted.
For numerical stability reason an empirical velocity filter is used when the velocity is larger than 5.0 ft/s (1.53 m/s) as follows:
v’ = 5.0 + 0.1*(v -5.0)
where v = velocity in ft/s
The typical headloss coefficients used in the Standard Method for estimating headloss through nodes are as follows;
HEC-22 Energy (Second Edition)
In this method various components such as plunging flow, benching, bend loss, initial headloss, entrance and exit losses are considered to determine the total headloss as per the FHWA’s Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22 (HEC-22).
The bend angle is considered in the initial headloss along with the equivalent diameter.
The equation used for computation of headloss coefficient “K” can be approximated as;
K = Ko * CD * Cd * CQ * Cp * CB
Where,
K = Adjusted headloss coefficient
Ko = Initial headloss coefficient based on relative junction size
CD =Correction factor for the pipe diameter
Cd = Correction factor for flow depth
CQ = Correction factor for relative flow
Cp = Correction factor for plunging flow
CB = Correction factor for benching
HEC-22 Energy (Third Edition)
This is an updated method developed by FHWA. In this, the headloss coefficient is not calculated based on correction factors as used in the second edition. Instead, the third edition computes total headloss by adding multiple individual headlosses.
The difference between the second and the third edition can be found here.
The equation for the headloss can be given as follows;
EGLa = Eai + HB + Hθ + Hp
Where,
Eai =Initial Access Hole Energy Level. See also: HEC-22 (Third Edition) Entrance Loss
HB =Additional loss for benching.
Hθ =Additional loss for angled inflows.
Hp = Additional loss for plunging flows.
The coefficient Hθ incorporates the bend angle for calculation along with the relative entrance loss.
AASHTO Method
In this method the coefficients for different kinds of bends are already available but can be changed based on user preferences. The table for Bend Angle vs. Bend Loss Coefficient (Kb) is available under calculation options, under the tab of Headloss (AASHTO).
Along with the bend loss, coefficients for other losses such as expansion, contraction etc. are also considered in computation.
HEC-22 Minor Loss Method
This method is generally used for computing headloss based on conservation of momentum as a lateral inflow enters the piped network. It takes the bend angle into consideration for calculation of total headloss. The equation is given as follows;
Except the Generic and the Absolute methods to compute headloss, all other methods as listed above consider the bend angle in calculating total headloss.
See Also
How are the coefficients derived for the HEC-22 structure loss method?
What are the differences between HEC-22 3rd edition and HEC-22 2nd edition headloss methods?